Enable nullability in Content.Server (#3685)

Content.Server/GameObjects/EntitySystems/AI/Pathfinding/Accessible/PathfindingRegion.cs

@@ -123,7 +123,7 @@ namespace Content.Server.GameObjects.EntitySystems.AI.Pathfinding.Accessible
         }
 
         // HashSet wasn't working correctly so uhh we got this.
-        public bool Equals(PathfindingRegion other)
+        public bool Equals(PathfindingRegion? other)
         {
             if (other == null) return false;
             if (ReferenceEquals(this, other)) return true;